Anisotropic Fluctuations Research Articles

Raman scattering model of the spin noise.

The mechanism of formation of the polarimetric signal observed in the spin noise spectroscopy (SNS) is analyzed from the viewpoint of the light scattering theory. A rigorous calculation of the polarimetric signal (Faraday rotation or ellipticity) recorded in the SNS is presented in the approximation of single scattering. We show that it is most correctly to consider this noise as a result of scattering of the probe light beam by fluctuating susceptibility of the medium. Fluctuations of the gyrotropic (antisymmetric) part of the susceptibility tensor lead to appearance of the typical for the SNS Faraday rotation noise at the Larmor frequency. At the same time, fluctuations of linear anisotropy of the medium (symmetric part of the susceptibility tensor) give rise to the ellipticity noise of the probe beam spectrally localized at the double Larmor frequency. The results of the theoretical analysis well agree with the experimental data on the ellipticity noise in cesium vapor.

On the Variation of Intermittency of Fast and Slow Solar Wind With Radial Distance, Heliospheric Latitude, and Solar Cycle

Intermittency, an important property of astrophysical plasma turbulence, is studied extensively during last decades from in-situ measurements of the solar wind plasma and magnetic field in the ecliptic plane and at higher latitudes, and heliocentric distances between 0.3 and 5 Astronomical Units. In this paper, we review the main findings on intermittency derived from investigation of solar wind turbulence for the inertial range of scales. It turns out that our current knowledge on the evolution of intermittency in the heliosphere is based on two missions, Helios two and Ulysses. We discuss the importance of data selection methodologies and applications for heliospheric spacecraft, the different data analysis techniques (the anomalous scaling of the structure function, the non-Gaussianity of the probability distribution functions, the local intermittency measure estimated from a wavelet representation and the multifractal spectrum). Studies show that Alvénic solar wind is less intermittent but reveals increase with the radial distance. Moreover, intermittency is stronger for the magnetic than for velocity fluctuations and is considered to be responsible for the increase with the radial distance of the anisotropy of magnetic fluctuations. The intermittency of fast solar wind at solar minimum decreases with latitude. Finally, the level of intermittency in the solar wind depends on solar cycle phase, reflecting the changes of the state of solar wind and suggesting that the deeper study of origin of fast and slow wind can further improve our understanding of the intermittency.

Uncinate fasciculus and its cortical terminals in aphasia after subcortical stroke: A multi-modal MRI study

Aphasia, one of the most common cognitive impairments after stroke, is commonly considered to be a cortical deficit. However, many studies have reported cases of post subcortical stroke aphasia (PSSA). The pathology and recovery mechanism of PSSA remain unclear. This study aimed to investigate PSSA mechanism through a multimodal magnetic resonance imaging (MRI) approach and a two-session study design (baseline and one month after treatment). Thirty-six PSSA patients and twenty-four matched healthy controls (HC) were included. All patients had subcortical infarctions involving left subcortical white matter for 1 to 6 months. The patients underwent MRI scan and Western Aphasia Battery (WAB) examination before and after one month’s comprehensive treatment. Region-wise lesion-symptom mapping (RLSM), tractography, fractional anisotropy (FA), and amplitude of low-frequency fluctuations (ALFF) analysis were conducted. After MRI preprocessing and exclusion, FA analysis included 35 patients pre-treatment and 16 patients post-treatment. ALFF analysis included 30 patients pre-treatment and 14 patients post-treatment. We found: 1) the amount of damage in the left uncinate fasciculus (UF) was associated with WAB aphasia quotient (AQ); 2) the left UF FA and left temporal pole (TP) ALFF were decreased and positively correlated with WAB-AQ, spontaneous speech, and naming in PSSA patients; and 3) PSSA patients showed increased left TP ALFF when their language ability recovered after treatment. The left TP ALFF change was positively correlated with AQ change. Our results demonstrate the importance of left UF and left TP (one of the cortical terminals of the left UF) in PSSA pathology and recovery. These results may further provide support for the disconnection theory in the mechanism of PSSA.

Anisotropic flow fluctuations of charged and identified hadrons in Pb-Pb collisions with the ALICE detector

Anisotropic flow fluctuations can be used to constrain the initial conditions and probe the properties and evolution of the system created in heavy-ion collisions. In this proceeding, the pT-differential flow coefficient vn with 2-, 4-, 6- and 8-particle cumulants for charged hadrons are investigated, which allows to study the higher order moments, including skewness and kurtosis, of v2 probability density function (p.d.f.). In addition, the v2 fluctuations of π±, K±, p+p¯, ϕ, KS0, Λ+Λ¯ and Ξ−+Ξ¯+ are also measured at mid rapidity in Pb-Pb collisions at sNN=5.02TeV cumulants. The implications of the results for understanding the properties of the medium are discussed.

Dataset for "Comparison between slow, anisotropic LE4PD fluctuations and the Principal Component Analysis modes of Ubiquitin"

Dataset for "Comparison between slow, anisotropic LE4PD fluctuations and the Principal Component Analysis modes of Ubiquitin"

Mössbauer studies of Zn0.05Fe2.95O4 nanoparticles

Nanoparticles of spinel ferrite (Zn0.05Fe2.95O4) were prepared using the process of high- temperature thermal decomposition (HTTD) and were treated with an Ar plasma for 30 min. The crystal structure was found to be cubic spinel with space group (Fd-3m). The lattice constants of Zn0.05Fe2.95O4 before and after the plasma treatment were found to be 8.3862 and 8.3853 A, respectively. The saturation magnetization value increased from 69.1 to 73.5 emu/g after the plasma treatment of the Zn0.05Fe2.95O4 nanoparticles. The Mossbauer spectra before and after the plasma treatment of Zn0.05Fe2.95O4 were analyzed at 4.2 K using four sextets. As the temperature was increased, the lines of the Mossbauer spectra broadened due to a fluctuation in the anisotropic hyperfine field. The Mossbauer spectra were found to have the same anisotropic field fluctuation of +H (P+ = 0.5) and −H (P− = 0.5). The relaxation frequency factor, f, was also calculated from the Mossbauer spectra.

Application of Helmholtz-Hodge decomposition and conditioned structure functions to exploring influence of premixed combustion on turbulence upstream of the flame

In order to explore the influence of combustion-induced thermal expansion on turbulence, a new research method is introduced. The method consists in jointly applying Helmholtz-Hodge decomposition and conditioned structure functions to analyzing turbulent velocity fields. Opportunities offered by the method are demonstrated by using it to process Direct Numerical Simulation data obtained earlier from two statistically 1D, planar, fully-developed, weakly turbulent, single-step-chemistry, premixed flames characterized by two significantly different (7.52 and 2.50) density ratios, with all other things being approximately equal. To emphasize the influence of combustion-induced thermal expansion on turbulent flow of unburned mixture upstream of a premixed flame, the focus of analysis is placed on structure functions conditioned to the unburned mixture in both points. Two decomposition techniques, i.e. (i) a widely used orthogonal Helmholtz-Hodge decomposition and (ii) a recently introduced natural Helmholtz-Hodge decomposition, are probed, with results obtained using them being similar in the largest part of the computational domain with the exception of narrow zones near the inlet and outlet boundaries. Computed results indicate that combustion-induced thermal expansion can significantly change turbulent flow of unburned mixture upstream of a premixed flame by generating anisotropic potential velocity fluctuations whose spatial structure differ substantially from spatial structure of the incoming turbulence. The magnitude of such potential velocity fluctuations is greater than the magnitude of the solenoidal velocity fluctuations in the largest part of the mean flame brush in the case of the high density ratio. In the case of the low density ratio, the latter magnitude is larger everywhere, but the two magnitudes are comparable in the middle of the mean flame brush. Contrary to the potential velocity fluctuations, the influence of the thermal expansion on the solenoidal velocity field in the unburned mixture is of minor importance under conditions of the present study.

Solar Wind Turbulence from 1 to 45 au. III. Anisotropy of Magnetic Fluctuations in the Inertial Range Using Voyager and ACE Observations

Abstract We examine both Voyager and Advanced Composition Explorer magnetic field measurements at frequencies that characterize the inertial range and evaluate the anisotropy of the fluctuations as they relate to both the compressive component and underlying wavevector anisotropy of the turbulence. The magnetic fluctuation anisotropy as it relates to the compressive component is directly dependent upon both the plasma beta of the thermal proton component and the ratio of magnetic fluctuation magnitude to the strength of the mean magnetic field. This has been seen before at 1 au. The magnetic fluctuation anisotropy in the plane perpendicular to the mean magnetic field, which is a measure of the anisotropy of the underlying wavevector distribution, should depend on the angle between the mean magnetic field and the radial direction and should be confined to values between one and the index of the power spectrum, which is typically 5/3. Our results show that the average of this anisotropy exceeds the value of the spectral index and is out of bounds with the theory. Although the results are suggestive of past analyses, we find that spherical expansion of the turbulence may offer at least a partial explanation of the apparent amplification of this measured anisotropy.

Anisotropy fluctuation and correlation in central α -clustered C12+Au197 collisions

In the framework of a multi-phase transport model, fluctuation and correlation of the azimuthal anisotropies in $^{12}$C+$^{197}$Au collisions at $\sqrt{S_{NN}}$ = 200 GeV are explored. Properties of the initial eccentricity and final harmonic flow fluctuation resulted from $^{12}$C+$^{197}$Au collisions with Woods-Saxon configuration and triangular $\alpha$-clustering configurations of $^{12}$C are investigated via scaled variance, skewness and kurtosis. Comparisons are made between results from $\alpha$-clustered configurations and Woods-Saxon configuration. The triangular flow fluctuation is found to have particular sensitivity in distinguishing triangular $\alpha$-clustering structure of $^{12}$C. Furthermore, correlations between initial eccentricities and final flow harmonics are studied with strong multiplicity dependence observed for the correlation functions.

Evolution of magnetic field in a weakly relativistic counterstreaming inhomogeneous e−/e+ plasmas

AbstractThe nonlinear evolution of electron Weibel instability in a symmetric, counterstream, unmagnetized electron–positron e−/e+ plasmas is studied by a 2D particle-in-cell (PIC) method. The magnetic field is produced and amplified by the Weibel instability, which extracts energy from the plasma anisotropy. A weakly relativistic drift velocity of 0.5c is considered for two counterstreaming e−/e+ plasma flows. Simulations show that in a homogeneous e−/e+ plasma distribution, the magnetic field amplifies exponentially in the linear regime and rapidly decays after saturation. However, in the case of inhomogeneous e−/e+ plasma distribution, the magnetic field re-amplifies at post-saturation. We also find that the amount of magnetic field amplification at post-saturation depends on the strength of the density inhomogeneity of the upstream plasma distribution. The temperature calculation shows that the finite thermal anisotropy exists in the case of an inhomogeneous plasma distribution which leads to the second-stage magnetic field amplification after the first saturation. Such density inhomogeneities are present in a variety of astrophysical sources: for example, in supernova remnants and gamma-ray bursts. Therefore, the present analysis is very useful in understanding these astrophysical sources, where anisotropic density fluctuations are very common in the downstream region of the relativistic shocks and the widely distributed magnetic field.

Evidence of weak antilocalization in quantum interference effects of (001) oriented La0.7Sr0.3MnO3–SrRuO3 superlattices

Quantum corrections to conductivity in the ferromagnetic La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) thin films depend on the structural mismatches and interfaces accommodating ions and their spins. Here, by making interfaces of LSMO and SRO in the form of artificial superlattices, we achieve positive magnetoresistance (MR) and weak antilocalization (WAL), although the individual component shows negative MR and weak localization (WL). The [20 unit cell (u.c.) LSMO/3 u.c. SRO]×15 superlattice stabilizes in tetragonal symmetry associated with the rhombohedral and orthorhombic structures and demonstrates the occurrence of the single magnon scattering process. The low-field MR of the superlattice fit to the Hikami–Larkin–Nagaoka expression yields 595 Å phase coherence length (lϕ) with WAL of carriers. As the SRO layer thickness in the superlattice increases to 5 u.c., the value of lϕ = 292 Å decreases, and positive MR increases confirm the manifestation of WAL by SRO. The orthorhombic symmetry of the SRO is preserved in the [20 u.c. SRO/3 u.c. LSMO]×15 superlattice, which shows the existence of locally cooperative bond-length fluctuations and conduction due to the scattering of the electron by the Fermi liquid electrons, bond length, and spin fluctuations. However, as the LSMO layer thickness in the superlattice is increased to 5 u.c., the WL effect suppresses WAL at the low field. The spin–orbit coupling associated with magnetic anisotropy, i.e., spin and bond length fluctuations, modifies the WL in the superlattices and leads to WAL, thereby achieving positive MR.

Large eddy simulation of the periodic cavity evolution and the turbulence characteristics around a Delft Twist-11 hydrofoil

In the present investigation, the LES WALE method along with the Zwart cavitation model is utilised to predict the cavitating flow around a Delft Twist-11 hydrofoil and the numerical results show a reasonable agreement with the experimental data. A novel elucidation is provided to the formation of the U-type structure and the Lumley method is introduced into the numerical simulation which can offer a validation of the simulation. The evolution of the cavity structure, especially the formation and development of U-type structures, was investigated thoroughly with the assistant of the Ω method, the Lamb vector, and the vorticity transport equation (VTE). As a third-generation vortex identification, Ω = 0.52 captures the vortex structure during the cavity evolution exhaustively. The cavitation-vortex interaction impacts the cavity evolution profoundly. The primary weak vortex and the secondary weak vortex both have a significant effect on the cavity shedding and the formation of the U-type structure. Moreover, the stretching action and the vorticity bearing motion described by the Lamb vector divergence clarify the formation of the special cavity structure in depth. In virtue of the mean velocity, Reynolds stresses, Lumley triangle, and Lumley distance, the turbulence characteristics were also investigated carefully. The cavitating motion plays a significant role in the turbulence velocity fluctuation and turbulence anisotropy around a twisted hydrofoil.

Effect of the intermittency dynamics on single and multipoint statistics of turbulent boundary layers

The instantaneous velocity field of turbulent flows is traditionally described in a statistical sense as a sum of an ensemble or temporal mean velocity at a specific location and a fluctuating term. While the Reynolds decomposition is well-established for fully turbulent flows, its applicability for flows that show intermittent behaviour is questionable. As intermittency is an integral part of turbulent boundary layer and turbulent jet and wake flows, an analysis of the effect of the intermittency on the statistical quantities is of fundamental interest. In this work, the effect of intermittency on single and multipoint statistics is studied for an experimentally obtained turbulent boundary layer data set at high Reynolds numbers by means of three different decomposition approaches. The analysis clearly shows that the Reynolds decomposition is only valid for single point statistics in the inner fully turbulent region of wall-bounded flows. In the wake region of a turbulent boundary layer, the Reynolds decomposition overestimates the turbulent kinetic energy due to the intermittency of the turbulent/non-turbulent interface. It is shown herein that these artefacts can be reduced or even completely avoided by applying either the boundary layer height or zonal based decomposition approach. In this study, the limitations of the different methods are analysed in detail and the approaches are evaluated comparatively by investigating statistical quantities such as turbulence intensity, anisotropy of fluctuations and characteristic length scales of coherent motions. The results make it possible to better understand and interpret the characteristics of the turbulent statistics of intermittent flows.

Coherence properties of high-frequency wave field propagating through inhomogeneous ionosphere with anisotropic random irregularities of electron density: 2. Further analysis and numerical results

Analytic theory, developed in the companion paper in order to describe the coherence properties of the high frequency wave field propagating through the stochastic transionospheric channel with the inhomogeneous background layer and anisotropic fluctuations of the electron density, is further analyzed extended and specified to obtain the numerical results, which illustrate the fields coherence properties for realistic parameters of the channel and geometries of propagation.

Coherence properties of high-frequency wave field propagating through inhomogeneous ionosphere with anisotropic random irregularities of electron density: 1. Theoretical background

Rigorous analytic solution is constructed for the first of 2 s-order space position and frequency coherence function (which is alternatively also termed as symmetric) of the high-frequency wave field propagating through the stochastic transionospheric channel with the inhomogeneous layer of the background ionosphere and anisotropic fluctuations of the electron density in the approximation of the appropriate Markov equation. It is obtained employing the anisotropic parabolic model of the effective structure function of the fractional electron density fluctuations with two different scales along and across the Earth's magnetic field. The explicit analytic solution is presented, which is capable of describing the coherence properties of the field for any given inhomogeneous distribution of the electron density in the background ionosphere along a path of propagation. The procedure of calibrating the spatial scales of the anisotropic parabolic model of the electron density fluctuations along and across the Earth's magnetic field lines in order to provide the adequate solution to correspond to the realistic anisotropic inverse power law spectrum of the electron density fluctuations is presented in the companion paper.

Cellular Contact Guidance Emerges from Gap Avoidance

SummaryIn the presence of anisotropic biochemical or topographical patterns, cells tend to align in the direction of these cues—a widely reported phenomenon known as “contact guidance.” To investigate the origins of contact guidance, here, we created substrates micropatterned with parallel lines of fibronectin with dimensions spanning multiple orders of magnitude. Quantitative morphometric analysis of our experimental data reveals two regimes of contact guidance governed by the length scale of the cues that cannot be explained by enforced alignment of focal adhesions. Adopting computational simulations of cell remodeling on inhomogeneous substrates based on a statistical mechanics framework for living cells, we show that contact guidance emerges from anisotropic cell shape fluctuation and “gap avoidance,” i.e., the energetic penalty of cell adhesions on non-adhesive gaps. Our findings therefore point to general biophysical mechanisms underlying cellular contact guidance, without the necessity of invoking specific molecular pathways.

Fully Coupled Two-Fluid Dynamics in Superfluid ^{4}He: Anomalous Anisotropic Velocity Fluctuations in Counterflow.

We investigate the thermal counterflow of the superfluid ^{4}He by numerically simulating three-dimensional fully coupled dynamics of the two fluids, namely quantized vortices and a normal fluid. We analyze the velocity fluctuations of the laminar normal fluid arising from the mutual friction with the quantum turbulence of the superfluid component. The streamwise fluctuations exhibit higher intensity and longer-range autocorrelation, as compared to transverse ones. The anomalous fluctuations are consistent with visualization experiments [Mastracci etal., Phys. Rev. Fluids 4, 083305 (2019)PLFHBR2469-990X10.1103/PhysRevFluids.4.083305], and our results confirm their analysis with simple models on the anisotropic fluctuations. This success validates the model of the fully coupled dynamics and paves the way for solving some outstanding problems in this two-fluid system.

Influence of He++and Shock Geometry on Interplanetary Shocks in the Solar Wind: 2D Hybrid Simulations

AbstractAfter protons, alpha particles (He++) are the most important ion species in the solar wind, constituting typically about 5% of the total ion number density. Due to their different charge‐to‐mass ratio, protons and He++particles are accelerated differently when they cross the electrostatic potential in a collisionless shock. This behavior can produce changes in the velocity distribution function (VDF) for both species generating anisotropy in the temperature, which is considered to be the energy source for various phenomena such as ion cyclotron and mirror mode waves. How these changes in temperature anisotropy and shock structure depend on the percentage of He++particles and the geometry of the shock is not completely understood. In this paper, we have performed various 2D local hybrid simulations (particle ions, massless fluid electrons) with similar characteristics (e.g., Mach number) to interplanetary shocks for both quasi‐parallel and quasi‐perpendicular geometries self‐consistently including different percentages of He++particles. We have found changes in the shock transition behavior as well as in the temperature anisotropy as functions of both the shock geometry and He++particle abundance: The change of the initialθBnleads to variations of the efficiency with which particles can escape to the upstream region facilitating or not the formation of compressive structures in the magnetic field that will produce increments in perpendicular temperature. The regions where both temperature anisotropy and compressive fluctuations appear tend to be more extended and reach higher values as the He++content in the simulations increases.

Novel electronic nematicity in heavily hole-doped iron pnictide superconductors

Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics), resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of nearly isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba1-x Rb x Fe2As2 iron-based superconductors. While for [Formula: see text], the nematic director points along the in-plane diagonals of the tetragonal lattice, for [Formula: see text], it points along the horizontal and vertical axes. Remarkably, for intermediate doping, the susceptibilities of these two symmetry-irreducible nematic channels display comparable Curie-Weiss behavior, thus revealing a nearly XY-nematic state. This opens a route to assess this elusive electronic quantum liquid-crystalline state.

Стохастический алгоритм генерации бездивергентного анизотропного однородного поля турбулентных пульсаций скоростей

Стохастический алгоритм генерации бездивергентного анизотропного однородного поля турбулентных пульсаций скоростей